1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus such as a line sensor which is constructed of unit cells, each containing a photodiode, arranged in a one-dimensional array and an area sensor which is constructed of unit cells arranged in a two-dimensional array, and in particular to a solid-state image pickup apparatus that offers optimum output in response to objects having a wide range of brightness.
2. Description of the Related Art
Solid-state image pickup apparatuses today have found applications in a diversity of fields. For example, video movie cameras and still video cameras use area sensors, and copying machines, facsimiles, and scanners employ line sensors. A line sensor is also used in the autofocusing device in a camera.
As digital technique today is in widespread use, these systems digitize the output from a solid-state pickup apparatus by means of an A/D converter before signal processing, and most of the A/D converters are of an 8-bit type. The 8-bit quantization is the minimum gradation required in handling image data. To achieve this level of gradation, the output of a sensor should appropriately match the input range of the A/D converter.
In copying machines, facsimiles, scanners and the like among the above applications, the position of an object is fixed and a constant light source is employed to illuminate the object. Thus, it is relatively easy to obtain the optimum sensor output that matches the input range of the A/D converter.
In video movie cameras, still video cameras, and the autofocusing device of camera, the brightness range of objects is wide. To adapt to the wide brightness range, diaphragm is controlled, exposure time (integrating time) is controlled by an electronic shutter, or the gain of a gain control amplifier disposed between the output of the sensor and the input of the A/D converter is controlled.
To achieve the above controlling, the brightness of an object should be accurately measured. To this end, Japanese Patent Application Laid-open No. Hei-4-255184 has disclosed the following solid-state image pickup apparatus. The disclosure is summarized as follows: out of the electron-hole pairs that are generated when light enters the solid-state image pickup apparatus, carriers having a polarity opposite to that of carriers accumulated in pixels are detected as a current, and that current is used as exposure information. FIG. 1 shows the construction of the disclosure in CCD-type solid-state image pickup apparatus.
Shown in FIG. 1 are a p-type substrate 101, an n-type diffusion layer 102 that forms photodiode of pixels, a p-type channel stop diffusion layer 103 for pixel isolation, a transfer CCD 104, and a control electrode 105 for the transfer CCD 104. The transfer CCD 104 is covered with a light cutting film 106, and lets in light to a pn-junction photodiode portion only.
In the CCD-type solid-state image pickup apparatus thus constructed, a substrate electrode 107 formed beneath the substrate 101 to keep the substrate 101 biased is electrically connected to the chip mount surface of the package of an image pickup device, and is further connected to one of the output pin terminals of the package. This terminal is typically connected to a power supply or ground to keep the substrate biased. In this example, however, to measure a current value, the terminal is connected to a current-voltage conversion amplifier 108, which constitutes exposure detecting means. The substrate electrode 107 is connected to the negative terminal of the current-voltage conversion amplifier 108. The negative terminal is biased at the same voltage as the positive terminal of the amplifier because of virtual ground. The output terminal 109 gives an output voltage V.sub.OUT expressed by the following equation (1). EQU V.sub.OUT =V.sub.SUB-( I.sub.P +I.sub.OF).multidot.R.sub.L ( 1)
where V.sub.SUB is the voltage of the substrate, I.sub.P is a photocurrent responsive to the quantity of incident light, I.sub.OF is an offset current that flows under dark conditions, and R.sub.L is the feedback resistor of the current-voltage conversion amplifier 108, As understood from the equation (1), the output voltage V.sub.OUT contains information indicative of the quantity of incident light. When the offset current I.sub.OF is sufficiently smaller than the photocurrent I.sub.P, the equation (1) is approximated by the following equation (2). EQU V.sub.OUT .apprxeq.V.sub.SUB -I.sub.P .multidot.R.sub.L ( 2)
Diaphragm, integrating time, or gain of the gain control amplifier may be controlled based on the output voltage V.sub.OUT that varies with the photocurrent I.sub.P.
The above disclosed solid-state image pickup apparatus suffers the following problems. First, the current-voltage conversion amplifier in FIG. 1 cannot be designed to be on-chip on a sensor that underwent standard manufacturing process. This is because although the substrate voltage V.sub.SUB in the solid-state image pickup apparatus in FIG. 1 should be biased at the lowest possible voltage relative to signal voltages handled on the chip, the output voltage V.sub.OUT is lower than the substrate voltage V.sub.SUB because the current due the output voltage is sunk. As a result, the output voltage V.sub.OUT is the one that cannot be handled on the sensor chip.
Second, the range of the power supply in the system is widened. The solid-state image pickup apparatus in FIG. 1 in its operation needs a positive power supply relative to the substrate voltage, while the current-voltage conversion amplifier needs a negative power supply. This widens the range of voltage required. The range of the power supply typically available in the standard system may not meet the voltage range requirement of FIG. 1.
Third, it is difficult to get outputs from blocks arbitrarily divided on the sensor. The above cited disclosure gives an embodiment, in which outputs are obtained from substrate electrodes divided into a plurality of portions as shown in FIG. 2. In FIG. 2, designated 111 are channel stop diffusion regions formed between a plurality of divided blocks, and designated 112 is an image pickup surface.
To achieve the object of the above cited embodiment, however, the current-voltage conversion amplifier should be completely free from offset voltage. If the current-voltage conversion amplifiers suffer from offset voltages, and the offset voltages are not balanced between the plurality of amplifiers, voltages relative to the substrate electrodes are different. Inter-block currents take place, leading to erroneous components. Blocks are not precisely divided. Therefore, block division turns out to be difficult.